What happened
On June 7, 2023, a CAP 232 aerobatic aircraft, registration F-HERA, was performing a sequence of maneuvers at approximately 3,000 feet near the Montluçon - Guéret aerodrome. After completing several figures from a planned aerobatic program, the pilot initiated a vertical climb. At an angle of roughly 45 degrees, the Plexiglas bubble canopy detached from the aircraft frame and shattered.
Fragments of the shattered canopy struck the pilot, causing minor injuries to the face. The pilot, wearing sunglasses which helped mitigate the impact, immediately reduced power and allowed the aircraft to climb briefly to inspect the condition of the tail unit. Finding the aircraft controllable, the pilot executed an adapted circuit and landed on runway 35 at a speed of 170 km/h. The aircraft sustained minor damage.
The investigation
The investigation focused on the structural integrity of the canopy attachment. Investigators found that nearly the entire canopy had shattered, with only two pieces remaining attached to the frame via three positioning screws. Five other screws were missing.
Technical analysis by CETIM revealed that the adhesive used to bond the Plexiglas to the frame had failed. The investigation established that the edges of the Plexiglas had not been properly prepared with the required sanding (grit-1200) before bonding. Furthermore, the adhesive used, TECNITE MSP15, has a very rapid skin-forming time of only 10 minutes. It was determined that the parts were likely not joined within this critical window. Additionally, the adhesive used was not among those recommended by the manufacturer, and the maintenance documentation provided to the workshop lacked specific guidance on approved adhesives.
Findings
- The primary cause of the separation was an adhesive failure between the Plexiglas bubble and the frame.
- The Plexiglas surface was not properly sanded prior to the installation of the canopy.
- The adhesive used had a much shorter skin-forming time than typical for such applications, and the workshop exceeded this window during assembly.
- The maintenance workshop lacked access to a list of recommended adhesives from the manufacturer.
- The pilot had previously noted loose positioning screws, which likely served as an unobserved indicator of the degrading adhesive bond.